Recently in Japan, physicians used MRI angiography (MRA) for the detection of asymptomatic intracranial diseases. Asymptomatic unruptured intracranial aneurysms (UIAs) are one of the most common abnormalities; usually the patients are advised to undergo preventive surgery. We evaluated the effectiveness of preventive surgery for asymptomatic UIAs, using published data in the literature with decision analysis on a Markov model. The baseline analysis showed that preventive surgery is superior to follow-up in a wide range of patient ages, but only by small QALY margin. Sensitivity analyses showed that the rate of rupture and the clinical outcome of surgery will influence the choice.